drm/exynos: stop using sgtable in page fault handler

Already struct exynos_drm_gem_buf has pages of the buffer when buffer is
created, so just can use pages in page fault handler, we don't have to
make sgtable of the buffer. But this needs to construct pages of the
buffer that is imported from dma-buf prime.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>